Efforts to improve upon the performance of natural mineral oil based lubricants by the synthesis of oligomeric hydrocarbon fluids have been the subject of important research and development in the petroleum industry for a large number of years and have led to the introduction of a number of superior polyalpha-olefin (PAO) synthetic lubricants produced by the oligomerization of alpha-olefins or 1-alkenes. In terms of lubricant property improvement, the thrust of the industrial research effort on synthetic lubricants has been toward fluids exhibiting useful viscosities over a wider range of temperature, i.e., improved viscosity index (VI), while also showing lubricity, thermal and oxidative stability and pour point equal to or better than mineral oil. These new synthetic lubricants exhibit lower friction characteristics and are therefore capable of increasing mechanical efficiency of various types of equipment including engines, transmissions, worm gears and traction drives, doing so over a wider range of operating conditions than mineral oil lubricants.
Notwithstanding their generally superior properties, PAO lubricants are often formulated with additives to enhance those properties for specific applications. Among the more commonly used additives are oxidation inhibitors, rust inhibitors, metal passivators, antiwear agents, extreme pressure additives, pour point depressants, detergent dispersants, viscosity index (VI) improvers, foam inhibitors and the like. This aspect of lubricant technology is described in Kirk-Othmer "Encyclopedia of Chemical Technology", 3rd Edition, Vol. 14, pp. 477-526, to which reference is made for a description of the use of such additives.
Improvements in synthetic lubricant technology have resulted both from new additive developments intended to address deficiencies in the lubricant (oligomer) basestocks as well as from developments in new base fluid (oligomer). Recently, lubricant compositions (referred to in this specification as HVI-PAO) of remarkable high VI coupled with low pour point have been developed. These lubricant compositions are described in U.S. Pat. Nos. 4,827,064 and 4,827,063, to which reference is made for a detailed description of these lubricants, methods for their preparation and of their properties and uses. These HVI-PAO materials comprise polyalpha-olefin oligomers prepared by the use of a reduced metal oxide, preferably reduced chromium, oligomerization catalyst. The lubricant product is characterized by a branch ratio less than 0.19, indicating a high degree of linearity and pour point below -15.degree. C. In its as-synthesized form, the HVI-PAO oligomer has olefinic unsaturation associated with the last of the recurring monomer units in the structure and this can be removed by a simple hydrogenative treatment to produce a stabilized, fully saturated oligomer product. Lubricants produced by the process cover the full range of lubricant viscosities and exhibit a remarkably high VI and low pour point even at high viscosity. Products of higher viscosity can also be produced by operating the oligomerization process at lower temperatures, typically -20.degree. to +90.degree. C., and these high viscosity products are useful as lubricant additives, especially VI improvers for both mineral and synthetic oils, as described in co-pending application Ser. No. 07/345,606, filed May 1, 1989, to which reference is made for a description of these higher viscosity oligomers, their properties and uses and of the method by which they may be made.
The process for preparing the HVI-PAO lubricants comprises, as noted above, contacting a C.sub.6 -C.sub.20 1-alkene feedstock with reduced valence state chromium oxide catalyst on porous silica support under oligomerizing conditions in an oligomerization zone to produce the high viscosity, high VI liquid hydrocarbon lubricant with branch ratios less than 0.19 and pour points below -15.degree. C. The oligomerization temperature is typically maintained at a value between 90.degree. and 250.degree. C. to produce the lubricant viscosity products. By operating the oligomerization process at lower temperatures, however, higher viscosity materials may be produced and these materials may be used as viscosity index (VI) improvers for lubricants, both of mineral oil and synthetic origin, as described application Ser. No. 07/345,606, filed May 1, 1989. The higher viscosity HVI-PAO products described in application Ser. No. 07/345,606 typically have viscosities between 725 and 15,000 cS at 100.degree. C., corresponding to weight molecular weights from about 15,000 to 200,000 and number molecular weights from about 5,000 to about 50,000; carbon numbers for these molecular weights are from about C.sub.30 to about C.sub.10,000, with a preferred range from about C.sub.30 to about C.sub.5,000. Like the liquid lubricant oligomers, these higher molecular weight oligomers are characterized by high VI coupled with excellent low temperature fluidity properties including pour point for the liquid products.
Although these HVI-PAO oligomers have excellent fluid flow properties, as evidenced by their high VI values and low pour points, they do not necessarily possess the highest degree of thermal and oxidative stability under the most stringent conditions. For this reason, it would be desirable to improve their stability in these respects if this could be done without significant deterioration of their excellent rheological characteristics.
Alkylated aromatics, particularly alkylated naphthalene are known to possess good thermal and oxidative stability as disclosed in U.S. Pat. Nos. 4,211,665, 4,238,343, 4,604,491 and 4,714,7944 but these naphthalene derivatives do not usually possess good rheological properties: in particular, they have extremely poor VI, consonant with their aromatic character. The antiwear properties of alkylnaphthalene lubricating fluids are described in Khimiya i Tekhnologiya Topliv i Masel, No. 8, pp. 28-29, August, 1986 and are reported to show some promise as base stocks for lubricants. In general, however, alkylated naphthalenes have been disappointing as lubricants although their good thermal and oxidative stability have made them suitable for use as transformer oils and heat exchange media.
In co-pending application Ser. No. 07/293,911, filed January 6, 1989, there is described a method of improving the thermal and oxidative stability of the HVI-PAO oligomers by alklating the unsaturated oligomer product with an aromatic compound such as benzene or naphthalene. The products have the enhanced stability and good solvency characteristics associated with the aromatic component while retaining the excellent rheological characteristics of the HVI-PAO oligomers. According to the method described in application Ser. No. 07/293,911, the HVI-PAO oligomer is reacted with the aromatic compound in the presence of an alkylation catalyst such as a Lewis acid e.g. aluminum trichloride or born trifluoride or a solid acidic zeolite such as zeolite Y.